Kokko,
Welcome to the forum.
A couple of ideas;
You can get large plastic (PTFE?) washers at home improvement stores, that would fill the gaps and should grip the rudder well enough.
You could use some hardwood shims and epoxy or polyester resin to build up the rudder head in the area inside the rudder head.
Some of us drill a 1/4" hole through the rudder and head and put in a wooden dowel to keep the rudder down. This would still break away if you hit something, but otherwise the rudder is perminately in the full down position. This is class legal for racing. no more than 1/4" and must be wood.
If you are a bit handy, you may also consider building a new rudder that is thicker.
happpy sailing phill
